The radio frequency (RF) band of the electromagnetic spectrum contains frequencies from approximately 3 kilohertz (3,000 hertz, or 3 kHz) to 300 gigahertz (GHz). In many places, the use of this band is regulated by the government. For example, in the United States, the broadcast television system is disseminated via radio transmissions on designated channels in the band from 54 megahertz (MHz) to 890 MHz, so that separate modulated frequencies are used to carry different TV shows concurrently in the allocated spectrum. There are two radio frequencies at which satellites broadcast signals in the Global Positioning System (GPS); L1 signals are broadcast at 1.57542 GHz, and L2 signals at 1.2276 GHz. Signals such as these are typically modulated signals that each contain a selected channel of data; the frequencies carrying such signals are typically referred to as “carrier frequencies.”
Some radio receivers, such as super-heterodyne receivers, operate by converting a signal considered to be a “high” RF signal to a signal of a lower frequency, often referred to as an intermediate frequency (IF) by mixing the RF signal with a mixing signal of a different frequency, to allow for more convenient amplification and selection of the desired channel (and digitization as discussed below). The difference between the frequencies of the RF signal and the mixing signal is the frequency of the IF signal. The mixing signal is typically provided by a local oscillator in the receiver, and the combination of a local oscillator and a mixer is commonly referred to as a “down-converter.”
(As used herein, a receiver that receives any signal in the RE band is a radio receiver, even if the signal is a television or GPS signal as above. Those of skill in the art will appreciate that “RF signal” is now often used to refer to a received signal that has not been down-converted, and that “IF signal” is used to refer to a down-converted signal, even though the RF band may technically cover the frequency of a down-converted IF signal.)
Signals in the television spectrum ma be down-converted in this way so that, for example, an RE signal in the 500 MHz to 506 MHz region (which is TV channel 19 in the United States) may be down-converted to an IF signal in the 41 MHz to 47 MHz region by mixing the RF signal with a mixing signal of 459 MHz (since 500 MHz−459 MHz=41 MHz). Other television signals, or the GPS signals described above, may be similarly down-converted. Amplification and selection of the channel to be received can thus occur in the IF frequencies, which are more easily operated upon than the higher RF frequencies.
In modern radio receivers, channel selection and recovery of the data in the channel is performed by converting the down-converted IF signal into the digital domain. An analog-to-digital converter (ADC) is used to transform the analog IF signal into a digital data stream after which sophisticated digital signal processing (DSP) techniques can be used to recover from noise, dropout and similar artifacts of as digital radio system. As is also known in the art, to convert an analog signal to a digital signal, an ADC must sample the analog signal at a rate at least twice as fast as the frequency of the signal itself. Thus, down-converting also allows the use of slower, and less expensive, ADCs.
In recent years, “multichannel communications systems” in which multiple distinct channels are each used to carry a separate received analog or digital data stream have become common. Such a system typically requires separate down-conversion of each channel from its original RF frequency to the IF band. This may be done, for example, by multiple instances of known down-converters as above, each including a local oscillator and mixer; two or more down-converters may be used and each may down-convert a different channel of information.
For example, a typical television set-top box may be able to simultaneously receive and process two channels by having two local oscillator-mixer combinations, each processing a different selected channel, so that one channel may be watched while the other is recorded to memory (or both may be recorded). Some set top boxes have the capability of processing more than two channels, by increasing the number of local oscillator-mixer combinations. The frequency of the local oscillators may be adjustable to allow for down-conversion of channels of different frequencies.
An alternate way to achieve such down-conversion of multiple signals from arbitrarily selected channels at RF frequencies to a common IF band is by use of down-conversion technologies that use a particular method of sampling the RF signals. U.S. Pat. No. 7,028,070 (“the '070 patent”), entitled “High Speed Filter” (as well as the continuation of the '070 patent, U.S. Pat. No. 8,001,172) describes an alternative way of implementing a down-conversion by using a series of sampling elements activated in a “round-robin” fashion. One embodiment using such round-robin sampling to down-convert multiple RF channels simultaneously may be found in U.S. patent application Ser. No. 13/668,253 (“the '253 application,” now U.S. Pat. No. 8,693,972), entitled “Down-Conversion of Multiple RF Channels.” The '070 patent and '253 application are both commonly owned by the assignee of the present application, and are incorporated herein by reference as though set forth in full.
Such newer technologies which inherently enable multiple down-conversion may be preferable in comparison to multiple instances of a local oscillator and mixer, due to a reduced number of elements and thus potentially lower cost, they result in the same output, i.e., output signals in the IF band that contain the selected channels. However, each down-converted signal must typically be digitized and processed separately from the others, potentially requiring multiple or more expensive components, and more power, in order to process multiple signals simultaneously. While the number of components, the space needed to house them, and their power consumption may be of little concern in a set-top box, as communication systems continue to become mobile these issues will become more important.
It would be beneficial to be able to combine multiple down-converted signals into a single output signal, and to minimize the bandwidth of that output signal, in order to minimize the cost and complexity of the components needed to further process the down-converted signals, as well as to lower the power required by those circuits.